Recently, in high-frequency applications using a microwave band or a milliwave band as a carrier, for example, in wireless LAN or various communication terminals, reduction in size and thickness of equipment and circuit board has been demanded. In a circuit board for such high-frequency applications, filter elements such as a low-pass filter (LPF), a high-pass filter (HPF) and a band-pass filter (BPF) are designed with a distributed constant, for example, using a microstrip line or a strip line that enables relatively high space-saving, instead of using a lumped constant design using chip components like an inductor and a capacitor.
For example, a circuit board 100 shown in FIG. 1 has a BPF 101 of a flat structure, as a filter element designed with a distributed constant. In this circuit board 100, conductor patterns 103 made of copper or nickel plated with gold are formed as microstrip lines on a dielectric board 102 such as a printed board or a ceramic board, thus constituting the BPF 101. On the entire back side of the dielectric board 102, a ground part (not shown) is formed.
With such a BPF 101, it is possible to selectively transmit a signal of a desired frequency band by optimizing the shape of the conductor patterns 103. Since this BPF 101 is a part of the whole pattern wiring formed on the dielectric board 102 and has a flat structure, the BPF 101 can be collectively formed when forming the pattern wiring on the dielectric board 102, for example, by print processing, lithography processing or the like.
In the circuit board 100 shown in FIG. 1, since the BPF 101 has a flat structure and the conductor patterns 103 are arrayed with an overlap of substantially ¼ of a passing wavelength λ, the length of the conductor patterns 103 is prescribed by the passing wavelength λ. In the circuit board 100, the conductor patterns 103 need to have a certain length and it is difficult to reduce the occupied area of the conductor patterns 103. Therefore, area-saving is limited.
Thus, in a circuit board 110 shown in FIGS. 2A to 2D, it is proposed to save the area by using a BPF 111 as a filter element that requires a smaller occupied area. This BPF 111 has a so-called tri-plate structure, which is a three-layer structure in which resonator conductor patterns 113 arranged substantially parallel to each other are formed in an inner layer of a multilayer board 112 such as a multilayer printed board.
Specifically, in the BPF 111, feeder wirings 114 are connected to substantially central parts in the longitudinal direction of the two resonator conductor patterns 113, as shown in FIG. 2C. The resonator conductor patterns 113 are held between two ground parts 116a, 116b as ground conductors, with dielectric layers 115 provided between the resonator conductor patterns 113 and the ground parts 116a, 116b, as shown in FIG. 2A. In this BPF 111, the two ground parts 116a, 116b are connected with each other in the form of interlayer connection by via-holes 117 and shield the resonator conductor patterns 113 in the layer. In the BPF 111, each of the two resonator conductor patterns 113 has a length that is substantially ¼ of the passing wavelength A, indicated by an arrow M in FIG. 2C. One end of each resonator conductor pattern 113 is connected to the via-hole 117 and the other end is opened. In this BPF 111, when shown in the form of an equivalent circuit as shown in FIG. 3, parallel resonance circuits are capacitive-coupled. Specifically, a parallel resonance circuit PR1 including a capacitor C1 and an inductance L1 connected between one of the two resonator conductor patterns 113 and the ground parts 116a, 116b, and a parallel resonance circuit PR2 including a capacitor C2 and an inductance L2 connected between the other of the two resonator conductor patterns 113 and the ground parts 116a, 116b, are capacitive-coupled via a capacitor C3.
Meanwhile, in the above-described circuit board 110, it is possible to reduce the area of whole body by reducing the occupied area of the filter element. However, when a semiconductor component 118 such as an IC or a chip component is mounted on the major surface, as shown in FIG. 4, the thickness of the whole body indicated by an arrow t1 in FIG. 4 is increased.
To solve this problem, it is proposed to reduce the thickness of the circuit board 110 indicated by an arrow t2 in FIG. 5 and thus reduce the thickness of the whole body including the semiconductor component 118, as shown in FIG. 5.
To realize the reduction in thickness of the semiconductor component, the present applicant has proposed the techniques described in the Japanese Publications of Laid-pen Patent Application Nos.2001-44704, 2001-44705 and 2001-44706. According to the technique described in these publications, if the thickness of the circuit board 110 is reduced, that is, if the thickness of the dielectric board 115 is reduced, the degree of electromagnetic coupling between the resonator conductor patterns 113 might not be sufficient, which affects the passing characteristic at the time when an electric signal passes through the BPF 111. Therefore, in the circuit board 110 with the reduced thickness, the loss within the passband of the BPF 111 is increased and the frequency bandwidth is reduced, making it difficult to acquire a desired filter characteristic.